Unmanned Aerial Vehicles (UAV) have become relatively common among hobbyists, commercial entities (e.g., aerial photography), and military users. These aerial vehicles generally operate at low altitudes where air traffic is busiest and most unpredictable. For example, take-off and landing of commercial aircraft, test flights, private pilot activity, hobbyists, balloons and blimps, aerial advertising, float planes, emergency responders and other UAVs may be more likely to be present within the UAV's airspace. A UAV, operating autonomously or under the control of an operator must actively avoid interference with other objects, both moving and stationary, that are present within the UAV's airspace.
Aircraft collision avoidance systems (ACAS) and aircraft separation assurance systems (ASAS) are intended to operate independently of a ground-based air traffic controllers. Several systems are commonly used onboard manned aircraft to avoid collisions and maintain aircraft separation, for example, airborne radar and traffic collision avoidance systems. However, these systems are often heavy, expensive, and/or rely on active interrogation of the transponder of aircraft in the vicinity of the aircraft conducting the interrogation. Lighter systems are generally passive and rely on transmission of transponder information from nearby aircraft, thereby only passively preventing an interaction between aircraft in the vicinity of the transmitting air vehicle. In some instances, an object may not be equipped with a transponder and therefore would be invisible to passive detection using these techniques. Additionally, in the busiest and most unpredictable airspace, i.e., low altitudes, manned air vehicles typically rely on the pilot and Air Traffic Controllers to prevent interactions and maintain adequate separation between aircraft.